Various woven and nonwoven sheet materials used in making medical drapes, medical gowns and absorbent articles, such as diapers and sanitary napkins, must be comfortable, soft, pliable and substantially liquid impermeable. The sheet materials used in medical apparel and absorbent articles function to contain the discharged materials and/or to isolate these materials from the body of the wearer or from the wearer's garments and bed clothing. As used herein, the term “absorbent article” refers to devices which absorb and contain body exudates, and, more specifically, refers to devices which are placed against or in proximity to the body of the wearer to absorb and contain the various exudates discharged from the body. Absorbent articles include disposable diapers, incontinence briefs, incontinence undergarments, incontinence pads, feminine hygiene garments, training pants, pull-on garments, and the like.
An ideal sheet material for use in medical apparel and absorbent articles will exhibit a high moisture vapor transmission rate that will reduce the build up of heat and humidity inside garments and articles made from the material. The ideal sheet material will also exhibit excellent barrier properties so as to prevent the passage or seepage of fluids, and will even prevent the passage of bacteria and viruses. The ideal material must also be strong enough so that it does not rip or delaminate under normal usage conditions regardless of whether the material is dry or wet. Where the sheet material is to be used in apparel, it is also important that the material be flexible, soft and drapable. Finally, where the sheet material is to be used in medical apparel, it is important that the sheet not generate fiber lint that might contaminate a medical environment.
PCT Publication No. WO 97/45259, which is hereby incorporated herein by reference, discloses a breathable composite sheet material comprised of a moisture vapor permeable thermoplastic film adhered to a fibrous substrate. The breathable thermoplastic film is primarily comprised of a polymer material selected from the group of block copolyether esters, block copolyether amides and polyurethanes. The fibrous substrate is a nonwoven sheet made primarily of a polymer fibers that are not compatible with the film, such as a polyolefin fibers. The film is adhered to the fibrous substrate by extruding a layer of the molten film-forming polymer directly onto the fibrous substrate and then mechanically engages the film and the fibers of the substrate, as for example by pressing the molten film into the fibrous substrate in a nip formed between two rolls.
U.S. Pat. No. 5,445,874 discloses a waterproof, blood-proof and virus-proof laminate material suitable for use in protective apparel. The laminate is comprised of a moisture vapor permeable film adhered to a woven or nonwoven fabric. The preferred film is a thermoplastic polyester elastomer. The disclosed fabrics include nonwoven fabrics of polyester, nylon and polypropylene. U.S. Pat. No. 5,445,874 discloses that the film can be laminated to the fabric by powder adhesive lamination, hot melt lamination, or wet adhesive lamination.
Adhesive lamination, thermal lamination and extrusion coating methods have all been used to produce composite sheets of a fibrous nonwoven substrate and a moisture vapor permeable, substantially liquid impermeable film. It has been possible to make such composite sheets with good barrier properties so long as the moisture vapor permeable film is relatively thick (i.e., >25 microns). However, it has been difficult to make such composite sheets with thinner films without sacrificing important barrier properties. Very thin moisture vapor permeable films are desirable in a composite sheet because thinner films facilitate greater flux of moisture vapor through the composite sheet and because thinner films use less of the film material and are accordingly less expensive to produce.
Adhesive lamination is carried out in a post film formation step. For adhesive lamination to be feasible, the moisture vapor permeable film must have enough tensile strength and tear strength so that the film can be formed, wound onto a roll, and later unwound and handled during the adhesive lamination process. It is difficult to handle moisture vapor permeable films less than 25 microns (1 mil) in thickness during the adhesive lamination process without tearing the film or introducing defects into the film.
Thermal lamination of moisture vapor permeable films less than 25 microns thick has similarly resulted in composite sheet materials with inadequate barrier properties. When composite sheets are made by thermally laminating a thin film to a fibrous substrate, the thin film handling problems associated with adhesive lamination as described above are encountered. In addition, to carry out a thermal lamination, the film must be subjected to elevated temperatures and pressures so as to soften the film and force it into mechanical engagement with the fibrous substrate. Generally, the peel strength between the film and the fibrous substrate increases with increasing lamination temperatures and increasing nip pressures. Unfortunately, when moisture vapor permeable films with a thickness of less than 25 microns are subjected to the increased temperatures and pressures needed to obtain adequate peel strength in the composite sheet, small holes develop in the film such that the composite sheet does not exhibit the fluid barrier properties desired in a composite sheet for use in absorbent articles or medical apparel. These defects can result from the non-uniform temperature throughout the web during bonding or from high nip pressures.
A composite sheet with excellent tensile and peel strength, that does not emit loose fibers, can be produced using a carded web of staple fiber that is powder bonded with an adhesive that is compatible with the fibers of the web. The composite sheet is produced by extrusion coating the powder-bonded web with a molten thin film that is also compatible with the fibers of the web and the powder adhesive. “Compatibility” of thermoplastic materials is an art-recognized term that refers, generally, to the degree to which the thermoplastic materials are miscible and/or interact with each other. Similarly, “incompatible” materials, as used herein, means polymer materials that are substantially immiscible or do not interact with each other. Incompatible materials do not wet or adhere well to each other, even when heated.
A composite sheet, made from a powder-bonded web that has been extrusion coated with a film of a thermoplastic polymer compatible with the fibers of the web and the solidified powder adhesive, exhibits good tensile strength and low linting because the solidified powder adhesive binds all of the fibers in the web into a strong matrix. These sheets exhibit excellent peel strength because the film readily adheres to the compatible adhesive and fibers of the web. For example, excellent tensile strength, peel strength and linting resistance can be obtained where the film, the nonwoven and the adhesive are all comprised of polyester polymers. Unfortunately, the film layer in composite sheets of this type is so thoroughly and completely bonded to the nonwoven that the sheet has a stiff paper-like feel that is unsuitable for apparel or many kinds of absorbent articles.
Accordingly, there is a need for a composite sheet material that acts as a barrier to fluids, bacteria and viruses, yet is also highly permeable to moisture vapor. Such a moisture vapor permeable, fluid impermeable composite sheet material should be durable, strong, and low linting, while at the same time being soft, flexible and comfortable enough for use in apparel products and absorbent articles. There is a further need for such a composite sheet that can be produced in an economical fashion, i.e., film extrusion and lamination in one process. There is a corresponding need for absorbent articles utilizing such materials to provide such characteristics.